leaves on my small trees are brownish like there is disease, or eaten with something.
brown shriveled bits appear on the leaves.
Certified GKH Gardening Expert
It would be hard to pin down without seeing the tree in question. I can take a few guesses and get you started in the right direction, though.
Usually, this is an indication that it was overwatered. This will lead to an infection which will show as you have described. If the conditions are not wet, or have not been wet, and you are sure that you tree has had a period to dry out thoroughly between waterings then you can rule this out. This article will help you with fungicide use to correct this: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Pests, such as biting insects can cause brown spotting and leaf problems. This will take a pesticide to correct. These articles will help: https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm
Last, could be a nutrient deficiency. In this case, it depends on how the spots show up, where they are located, and the pH of the soil. A soil test will help you to determine what the soil is lacking in.
This article will help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm